GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(2)
getpriority, setpriority - get/set program scheduling priority
int getpriority(int which, int who);
int setpriority(int which, int who, int prio);
The scheduling priority of the process, process group, or user, as indicated by which and
who is obtained with the getpriority() call and set with the setpriority() call.
The value which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is interpreted
relative to which (a process identifier for PRIO_PROCESS, process group identifier for
PRIO_PGRP, and a user ID for PRIO_USER). A zero value for who denotes (respectively) the
calling process, the process group of the calling process, or the real user ID of the
calling process. Prio is a value in the range -20 to 19 (but see the Notes below). The
default priority is 0; lower priorities cause more favorable scheduling.
The getpriority() call returns the highest priority (lowest numerical value) enjoyed by
any of the specified processes. The setpriority() call sets the priorities of all of the
specified processes to the specified value. Only the superuser may lower priorities.
Since getpriority() can legitimately return the value -1, it is necessary to clear the
external variable errno prior to the call, then check it afterward to determine if -1 is
an error or a legitimate value. The setpriority() call returns 0 if there is no error, or
-1 if there is.
EINVAL which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
ESRCH No process was located using the which and who values specified.
In addition to the errors indicated above, setpriority() may fail if:
EACCES The caller attempted to lower a process priority, but did not have the required
privilege (on Linux: did not have the CAP_SYS_NICE capability). Since Linux
2.6.12, this error occurs only if the caller attempts to set a process priority
outside the range of the RLIMIT_NICE soft resource limit of the target process; see
getrlimit(2) for details.
EPERM A process was located, but its effective user ID did not match either the effective
or the real user ID of the caller, and was not privileged (on Linux: did not have
the CAP_SYS_NICE capability). But see NOTES below.
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.
A child created by fork(2) inherits its parent's nice value. The nice value is preserved
The degree to which their relative nice value affects the scheduling of processes varies
across UNIX systems, and, on Linux, across kernel versions. Starting with kernel 2.6.23,
Linux adopted an algorithm that causes relative differences in nice values to have a much
stronger effect. This causes very low nice values (+19) to truly provide little CPU to a
process whenever there is any other higher priority load on the system, and makes high
nice values (-20) deliver most of the CPU to applications that require it (e.g., some
The details on the condition for EPERM depend on the system. The above description is
what POSIX.1-2001 says, and seems to be followed on all System V-like systems. Linux ker-
nels before 2.6.12 required the real or effective user ID of the caller to match the real
user of the process who (instead of its effective user ID). Linux 2.6.12 and later
require the effective user ID of the caller to match the real or effective user ID of the
process who. All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, 4.3BSD, FreeBSD 4.3, Open-
BSD-2.5, ...) behave in the same manner as Linux 2.6.12 and later.
The actual priority range varies between kernel versions. Linux before 1.3.36 had -infin-
ity..15. Since kernel 1.3.43, Linux has the range -20..19. Within the kernel, nice val-
ues are actually represented using the corresponding range 40..1 (since negative numbers
are error codes) and these are the values employed by the setpriority() and getpriority()
system calls. The glibc wrapper functions for these system calls handle the translations
between the user-land and kernel representations of the nice value according to the for-
mula unice = 20 - knice.
On some systems, the range of nice values is -20..20.
Including <sys/time.h> is not required these days, but increases portability. (Indeed,
<sys/resource.h> defines the rusage structure with fields of type struct timeval defined
According to POSIX, the nice value is a per-process setting. However, under the current
Linux/NPTL implementation of POSIX threads, the nice value is a per-thread attribute: dif-
ferent threads in the same process can have different nice values. Portable applications
should avoid relying on the Linux behavior, which may be made standards conformant in the
nice(1), renice(1), fork(2), capabilities(7)
Documentation/scheduler/sched-nice-design.txt in the Linux kernel source tree (since Linux
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2013-02-12 GETPRIORITY(2)